The invention is based on a tank venting device for motor vehicles having an internal combustion engine supplied from a fuel tank.
In a known tank venting device of this type (German Patent Disclosure DE 41 24 465A1), the metering of the air stream carried via the adsorption filter for filter regeneration is effected by a tank venting device, which is disposed between the air outlet of the filter housing and the intake tube of the engine and whose opening cross section is controlled as a function of engine operating parameters. Depending on the opening cross section of the tank venting device, and as a consequence of the negative pressure prevailing in the intake tube, a certain quantity of air is aspirated via the adsorption filter, and a corresponding regeneration mixing quantity is delivered to the engine. Since after all, depending on the operating state of the engine, the negative pressure in the intake tube fluctuates severely--for instance, in the upper load range of the engine or in engines without throttle valves there is no negative pressure available--precise metering of the regeneration mixture can be accomplished only with very great difficulty, or not at all.
To make a regeneration mixing quantity available even in internal combustion engines lacking a throttle valve and at full load in the intake tube, in an also-known tank venting device (DE 196 257 02 A1), the air feed pump required by a diagnosis device for error detection in order to generate an overpressure in the tank venting device connected to the fuel tank is utilized to generate the regenerating air flow. To that end, the air feed pump is connected to the air inlet of the filter housing via a bypass and is left on for the duration of filter regeneration. Once again, however, the metering of the air flow--and hence of the regeneration mixture--is effected by the controlled tank venting device located between the air outlet of the filter housing and intake tube of the engine. However, the capacity of the air feed pump must be limited, since when the regenerating air flow is generated air is blown through the fuel tank, and for safety reasons the pressure in the fuel tank must be kept below a maximum pressure of about 50 hPa. Increasing the air quantity without exceeding the maximum allowable tank pressure is possible only by means of a very large guide cross section between the air outlet of the filter housing and the tank venting device.